Draught beer.

[Galena]

CO2

Natural CO2 from fermentation or introduced to the pin? Does oxygen not enter when pouring or do they work like a wine box?

 

[foxy]

You can use the collapsible bags like a wine cask or introduce CO2 into the cask either from saved CO2 from the fermentation, or from a CO2 canister.

 

[trueblue]

So in a polypin there is no carbonation? What stops the oxidising?

Priming sugar

 

[foxy]

I am sure there are many innovative home brewers around who could save these from going to get recycled.
https://www.youtube.com/user/KeyKeg